In order to produce a liquid preparation of fat-soluble drugs in the past, there were adopted methods for preparing an aqueous solution by dissolving these drugs in a solubilizing agent such as alcohols or surface active agents. In particular, it is ordinary to use polyoxyethylene hydrogenated castor oils or polyoxyethylene glycol fatty acid esters as a surface active agent in the methods for dissolving the fat-soluble drugs. However, the amount of the fat-soluble drug which can be dissolved by these methods is insufficient, and the internal liquid preparation obtained thereby is uncomfortable in taste.
On the other hand, in order to contain a fat-soluble drug in an aqueous liquid preparation, there is widely used a method wherein the fat-soluble drug is dissolved in an oil and stirred together with an emulsifying agent to give an emulsion, thereby an aqueous phase and an oily phase are completely isolated from each other.
It is known that the stability of emulsions ordinarily increases with decreasing the particle size of emulsions. Japanese Patent Publication Nos. 88-61050-B and 94-57316-B disclose techniques for the preparations of emulsions with minute particles (microemulsion). However, these techniques have a drawback which, when a highly polar and fat-soluble drug is applied thereto, the particle size of the emulsion is increased, and the emulsion becomes unstable by heating, acids and ionic substances.
In addition, Japanese Patent Publication No. 95-23303-B discloses a microemulsion preparation containing a slightly water-soluble drug, however, the microemulsion described in the patent needs vigorous stirring at the time of production thereof, and it is impossible to apply a highly polar and fat-soluble drug thereto.